Due to a large number of formulations that may be used to prepare polyurethanes, polymers having a wide range of chemical and physical properties are possible. Physical properties can range from hard, rigid thermosetting materials to softer thermoplastic elastomers. Chemical properties may be tailored to control durability and chemical stability. Thus, polyurethanes have been extensively studied for use, and found wide applicability in medical applications, for example.
A number of thermoplastic polyurethanes exhibit a unique combination of biocompatibility, toughness, biostability, and surface functionality that has led to widespread use in implantable medical devices, such as pacemaker leads, blood bags, catheters, bladders, and artificial hearts. The elastomeric properties of certain polyurethanes also make them good potential candidates for applications where interaction with, and the mimicing of, soft tissue are desired. This is particularly true for vascular grafts.
Thermoplastic polyurethanes currently used in cardiovascular prosthesis mainly suffer from two drawbacks: (1) unproven long-term biostability; and (2) excessive dilation (plastic deformation) especially when used as porous implants. Both these negative aspects can be attributed to (among other factors) the weak nature of hydrogen bonding resulting in microphase separation.
Previous attempts to address the problem of improving the chemical and mechanical stability of thermoplastic polyurethanes include: (1) changing the soft-segment from ether to hydrocarbon or siloxane based materials; (2) modifying thermoplastic polyurethanes by reacting them with diisocyanates and unsaturated monomers in an extruder; or (3) modifying thermoplastic polyurethanes by blending them with acrylic monomers in an extruder to render them crosslinked. Although some degree of success has been achieved, these methods would not be useful in a medical application where very specific control over the modification and crosslinking is desired.